The present invention relates to a method and apparatus for encoding digital signals. The invention is of particular advantage in the compression and transmission of digital video signals.
Digital video compression systems can reduce the data rate of the transmitted signal by using temporal predictive coding. In the coding process, predictions are made for the reconstruction of the current picture (or frame) based on elements of frames that have been coded in the past. These picture elements (or blocks) can be manipulated in a certain way before they form the basis of the prediction. Prediction parameters (or motion vectors) relate the predicted blocks to earlier blocks. The motion vectors used in the manipulation of the blocks have to be transmitted to the receiver with a residual signal. This minimises the effects of prediction errors and makes it possible to reconstruct the sequence at the receiver.
Due to the spatial correlation of adjacent picture elements, the motion vectors themselves are coded differentially. This means that the differences between successive motion vectors are coded. Therefore small changes are represented by fewer bits of code than larger ones which are less common.
The quality of the block prediction is assessed using a distortion measurement which is based on the difference between the predicted block and the block to be coded.
A problem arises when predictions of approximately equal quality are made from completely different picture areas. There is a danger that a small advantage in prediction quality may be outweighed by a larger code word representing the motion vector. This situation is particularly common when picture areas are corrupted by noise and the motion vectors vary radically from one prediction to the next.
Accordingly, one object of the present invention is to provide a method and apparatus for improving the selection of motion vectors by using additional information in the motion estimator.
According to one aspect of the present invention, there is provided a method of generating prediction parameters for a digital coding technique, the method comprising the steps of: estimating a plurality of initial prediction parameters, storing the initial prediction parameters and re-estimating the or each initial prediction parameter in dependence on one or more other said initial prediction parameters, characterised in that the method further comprises repeating the re-estimating step one or more times to generate the prediction parameters.
According to a second aspect of the present invention there is provided apparatus for generating prediction parameters for a digital coding technique, the apparatus comprising: an estimator for estimating a plurality of initial prediction parameters, a buffer for storing the initial prediction parameters, and a re-estimator for re-estimating the or each initial prediction parameter in dependence on one or more other said initial prediction parameters, characterised in that the re-estimating step is repeated one or more times to generate the prediction parameters.
An advantage of the present invention lies in the fact that a set of motion vectors can be iteratively improved by using neighbouring (spatial or temporal) motion vectors in the prediction process. This effectively means that the motion vectors in the top left corner of the frame are influenced by motion vectors in the bottom right hand corner, or by motion vectors in future and past frames. As a result, the motion vectors represent the true motion in the video sequence and can be coded using fewer bits.
Advantageously the present invention can be used in motion compensated noise reduction techniques which require a good estimate of the underlying video motion for optimum performance. The present invention provides an estimate of this motion by being robust to the noise corruption which in the prior art causes neighbouring motion vectors to be radically different. Radically different motion vectors need a higher bandwidth for transmission.